1. Field of the Invention
The present invention relates to a post-processing apparatus having a buffer function of retaining a succeeding sheet while performing post-processing for a sheet having undergone image formation.
2. Description of the Related Art
There has been conventionally provided a system in which a post-processing apparatus (finisher) is connected downstream of an image forming apparatus such as a copying machine in the sheet conveyance direction of the image forming apparatus to perform post-processes such as stapling and punching. There is also proposed a post-processing apparatus which sequentially stacks sheets received from an image forming apparatus on an intermediate tray (to be referred to as a processing tray) arranged upstream of a stacking tray, and upon completion of stacking all sheets to form a booklet, performs post-processing such as stapling on the processing tray. A sheet bundle having undergone post-processing on the processing tray is discharged from the processing tray onto the stacking tray.
While performing post-processing (for example, stapling) for a preceding sheet bundle on the processing tray, some apparatuses overlay several succeeding sheets on the upstream side of the processing tray (to be referred to as buffering) to prevent the succeeding sheets from colliding with the sheet bundle during post-processing (Japanese Patent Laid-Open No. 9-48545). This arrangement in Japanese Patent Laid-Open No. 9-48545 prevents a decrease in image formation productivity when post-processing is executed. More specifically, in Japanese Patent Laid-Open No. 9-48545, a sheet is wound around a take-up roller arranged upstream of the processing tray for performing post-processing, and then the roller stops and waits. At the timing when a succeeding sheet arrives, the roller is driven again to overlay the wound sheet and the succeeding sheet. A predetermined number of sheets serving as a succeeding sheet bundle are overlaid, preventing discharge of the succeeding sheet bundle to the processing tray during execution of post-processing for a preceding sheet bundle on the processing tray. Post-processing can be done for a sheet bundle without widening the sheet conveyance interval in the image forming apparatus, and the productivity of the image forming apparatus does not decrease.
There is also proposed an apparatus which inhibits the buffering operation for a specific material and limiting the number of sheets to be overlaid in the buffering operation (U.S. Pat. No. 6,672,586). In U.S. Pat. No. 6,672,586, the buffering operation is inhibited or restricted for special sheets such as index paper, thick paper, and thin paper to prevent generation of a scratch or wrinkle of a sheet or generation of a jam owing to forced buffering of a special sheet.
A conventional post-processing apparatus executes buffering cancel processing. More specifically, when a buffering-inhibited sheet as disclosed in U.S. Pat. No. 6,672,586 is conveyed after a sheet capable of buffering, it temporarily waits till the end of post-processing for a preceding sheet bundle on the processing tray, and then the buffered sheet is discharged onto the processing tray. The image forming apparatus main body then discharges the buffering-inhibited sheet to the post-processing apparatus at a wider sheet interval between the buffering-inhibited sheet and a preceding one than that when a sheet is buffered. After that, the post-processing apparatus discharges the buffering-inhibited succeeding sheet onto the processing tray without buffering.
As a buffering arrangement, there has been conventionally proposed an arrangement which performs buffering by switch-back on a conveyance path, in addition to an arrangement which achieves buffering by take-up, as disclosed U.S. Pat. No. 6,672,586. In switch-back, after the trailing end of a sheet passes through the branch point between the conveyance path and a conveyance path for performing buffering, the sheet is conveyed in an opposite direction, guided to the buffering conveyance path by a path branch flapper or the like, and waits until the next sheet arrives. In apparatuses having these two exemplary arrangements, a sheet needs to be conveyed by a predetermined distance for buffering, and buffering itself takes a predetermined time. Depending on the post-processing time for a preceding sheet bundle and the productivity of the upstream image forming apparatus main body, the productivity may increase when it is controlled to convey sheets one by one without executing buffering, and discharge a succeeding sheet from the image forming apparatus main body in advance at a sheet interval corresponding to the processing time in sheet bundle processing on the processing tray.
Recently in the POD market, print jobs using various types of sheets coexistent in one bundle are frequently executed in form printing, transaction printing, and the like. The POD market requests high productivity. However, when performing the above-described operation, buffering processing is canceled in the overall image forming apparatus connected to the conventional post-processing apparatus, decreasing the productivity owing to the post-processing apparatus.